With a view to ensuring stabilization of the engine speed of an automotive engine during idling or the like, there have been proposed techniques in which pieces of engine speed information, which are repeatedly detected, are subjected to smoothing (averaging) processing, the smoothed engine speed so obtained is compared with the latest rotary speed information and the ignition timing is then corrected by data based on the difference therebetween. The ignition timing is advanced when the latest engine speed is lower than the smoothed value but is retarded conversely when the latest engine speed is higher than the smoothed value.
On the other hand, other proposals have also been made on the technique that in an automotive engine, intake and exhaust valves of some of its cylinders are kept closed or injectors for supplying fuel to some of its cylinders are selectively inactivated at the time of specific operation featuring a small required output torque such as under a low load to substantially stop a part of combustion strokes in the engine and hence to achieve an improvement in gas mileage, in other words, on variable displacement engines.
The present inventors are not aware of any case in which the above-described idle stabilization technique based on correction of the ignition timing is applied to the variable displacement engine described above. Even if the conventionally-known idle stabilization technique is applied simply to a variable displacement engine, the following problems arise.
First, upon switching part cylinder operation to full cylinder operation or vice versa at the time of idling, the engine speed generally varies (or the engine speed is deliberately changed to obtain an idle speed suited for part cylinder operation or full cylinder operation). At this switching, especially at the time of such switching that the engine speed varies upwards (or at the time of such switching that the engine speed is changed upwards), the ignition timing is corrected toward la retard side to suppress the increase in the engine speed. As a consequence, it takes time until the engine speed becomes close to a desired value subsequent to the switching, so that engine revolutions become unstable.
Where it is designed to deliberately make the idle speed higher at the time of part cylinder operation than at the time of full cylinder operation from the viewpoint of a countermeasure for vibrations (for example, where the idle speed is set at 850 rpm at the time of the part cylinder operation and at 750 rpm at the time of the full cylinder operation), there is the problem that the countermeasure for the vibrations is unable to show its effect, that is, to attain the primary object sufficiently immediately after the switching.
Further, as is shown in FIG. 11, the degree of a variation in the engine speed relative to a variation in the ignition timing at the time of idling differs between part cylinder operation and full cylinder operation. Described specifically, a variation in the engine speed relative to a variation in the ignition timing is smaller during the part cylinder operation than during the full cylinder operation and especially during the part cylinder operation, variations in the engine speed are leveled off in an advance-side range e1.
If a correction is attempted to stabilize idling during part cylinder operation by using an ignition timing correction gain Kinj set for full cylinder operation (Kinj is .DELTA..theta./.DELTA.Ne, where .DELTA..theta. is an ignition timing correction amount and .DELTA.Ne is a difference in engine speed), deviations in revolutions cannot be corrected fully because the correction is not adequate, for example, the absolute value of the ignition timing correction amount per unit revolution is small. This results in the problem that variations in the idle speed cannot be corrected with good responsibility by adjusting the ignition timing.
With the foregoing problems in view, the present invention has as an object thereof the provision of a control system for a multi-cylinder internal combustion engine, in which at the time of at least one of switching between full cylinder operation and part cylinder operation and the part cylinder operation, the engine speed subsequent to the switching between the full cylinder operation and the part cylinder operation can be promptly stabilized by controlling the manner of operation for the calculation of an ignition timing correction amount and the idle speed can also be stabilized with good responsibility by correcting the ignition timing.